Flocculation is a method of dewatering suspended solids by agglomerating the solids. Flocculation materially improves the dewatering rate of many types of suspended solids, including those used in mineral, papermaking, waste water treating and oil field applications.
Synthetic polymer flocculants have been utilized in the industry since the 1950's as flocculating agents in the treatment of suspended solids. However, due to modern concerns with environmental protection, sludge incineration, transportation and disposal costs, it has become increasingly desirable to improve on the performance of conventional linear polymer flocculants by providing a flocculating agent which achieves greater dewatering at a given polymer dose, or equivalent dewatering at a lower polymer dose.
The present invention provides compositions and methods for dewatering suspended solids, including those frequently encountered in the waste water treating, mining and papermaking industries, using high molecular weight, water-soluble or water-swellable, branched, cationic, polymer flocculants, as well as methods for making said compositions. The compositions and methods of the instant invention provide for superior dewatering when compared to those previously used in the art.
Linear polymer flocculants have been "structured" in the art through the use of branching or crosslinking agents. Polymer structuring is discussed by J. E. Morgan et al., Adv. Chem. Ser., Vol. 187, pp. 235-52 (1980). U.S. Pat. Nos. 4,720,346 and 4,943,378 describe the use of crosslinked cationic polymer particles having a dry particle size below 10 micrometers (.mu.m). U.S. Pat. Nos. 5,152,903 and 5,340,865 disclose a method of flocculating using cross-linked cationic polymer microparticles. U.S. Pat. No. 3,235,490 describes a flocculation method which utilizes crosslinked polyacrylamide. U.S. Pat. No. 3,968,037 teaches a method of releasing water from activated sewage sludge using crosslinked cationic emulsion polymers. Methods and compositions useful for thickening aqueous media are given in U.S. Pat. Nos. 4,059,552; and 4,172,066. Copending applications Ser. Nos. 08/028,916, 08/028,001, 08/437,258, 08/454,974 and 08/455,419, which are assigned to assignee of this invention and are all hereby incorporated herein by reference, describe methods for flocculating suspended solids using cationic, high molecular weight, water-soluble, branched polymers.
Water-soluble polymers may be characterized by determining the solution viscosity of dilute e.g. 0.05% to 1%, solutions of the polymers in pure water and in salt solutions. Herein, all percentages are given as weight percent based on total weight. The dilute solution viscosity of a linear, cationic, high molecular weight water soluble polymer is typically much higher in pure water than in, for instance, 1 molar (M) NaCl solution. For our purposes, the "bulk viscosity" of a polymer is defined as the viscosity of a 0.2% solution of polymer in pure water, measured using a rotating cylinder viscometer, e.g., Brookfield viscometer, under the conditions described in the Examples. As used herein, "standard viscosity" is the viscosity of a 0.1% solution of polymer in 1 M NaCl solution, also measured using a rotating cylinder viscometer, e.g., Brookfield viscometer, under the conditions described in the Examples. The ratio of the bulk viscosity to the standard viscosity, BV/SV, tends to vary as a function of the degree of structuring present in the polymer.
The "sedimentation value" also varies as a function of the degree of structuring present in the polymer. The "sedimentation value" is a sensitive indicum of the settling rate of a water-soluble or water-swellable polymer in salt solution. A sedimentation value of less than 10% means that there is little or no tendency for the polymer to sediment in salt solution. A sedimentation value is determined by preparing a 0.05% solution of a particular polymer in 0.001M NaCl, centrifuging part of the solution for about 60 minutes at about 18,000 X G (gravity) and 22.degree. C., and measuring the ultraviolet (UV) absorbance, at 215 nanometers (nm), of the uncentrifuged part and of the supernatant of the centrifuged part. The absorbance of the supernatant of the centrifuged part compared to the absorbance of the uncentrifuged part is calculated as .DELTA.A(uncentrifuged)--.DELTA.A(centrifuged)!/.DELTA.A(uncentrifuged), where .DELTA.A=A(polymer solution)-A(water) and A is the measured UV absorbance at 215 nm. The value calculated thereby is multiplied by 100 to give the sedimentation value, which is expressed as a percentage.
Surprisingly, it has now been found that water-soluble polymers having BV/SV of from about 300 to about 500 and having sedimentation values of less than 10%, are superior flocculants for suspended solids. In particular, these polymers give faster dewatering of waste activated sludge, in particular extended aeration activated sludge, than polymers of similar molecular weight and cationicity that do not have the BV/SV and sedimentation values mentioned above.